1. Field of the Invention
The present invention relates generally to a broadband wireless communication system, and more particularly, to an apparatus and a method for estimating a channel in the broadband wireless communication system.
2. Description of the Related Art
A transmitter and a receiver in a wireless communication system communicate with each other over a radio channel. According to characteristics of the radio channel, a signal passing through the radio channel may be distorted and deformed. Thus, the receiver measures a magnitude change and a phase change of the signal caused by the radio channel, and detects the signal transmitted by the transmitter using the measurements. Herein, the measurement of the magnitude and phase changes of the signal within the radio channel is referred to as channel estimation.
When utilizing an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme, which is a signal scheme for a physical layer of an advanced communication system, channel estimation mostly uses pilot signals inserted to specific locations. The pilot signals are generally classified as scattered pilots and continual pilots, based on their mapping scheme.
FIG. 1 illustrates an example of a pilot signal pattern in a conventional broadband wireless communication system.
Referring to FIG. 1, pilot signals continuously arranged along the time axis are the continual pilots 110, and pilot signals arranged at intervals along the time axis are the scattered pilots 120. When a channel is estimated using the pilot signals as illustrated FIG. 1, channel values of tones of the pilot signals are determined by the pilot signals and channels values of other tones are determined using interpolation.
The interpolation is conducted in both of a time direction and a frequency direction. The frequency-directional interpolation relies heavily on a recent Discrete Fourier Transform (DFT) based interpolation. The DFT based frequency-directional interpolation can reject considerable noise between Channel Impulse Response (CIR) components apart from each other, which cannot be removed in a Finite Impulse Response (FIR) filter type interpolation, and thus enhance the performance under much multipath delay. With respect to each CIR peak component, the DFT based frequency-directional interpolation enables filtering of the frequency response with sharpness over a possible level in the FIR filtering and narrow bandwidth, thus maximizing noise suppression effect. The DFT based frequency-directional interpolation can directly filter some image components still remaining between the original CIR components, which are not completely removed because of the inaccurate time-directional interpolation in the high-speed mobile reception environment, thus enhancing the reception performance of the high-speed mobile environment. Although the significant CIR component and the image component overlap in the long multipath environment, when their individual peak does not overlap, they can be filtered individually. Thus, it is possible to overcome the theoretical rate regulation according to the time-directional sampling interval by omitting the time-directional interpolation.
However, the output of the DFT based frequency-directional interpolation has a distortion problem called an edge distortion. That is, a sharp rectangular CIR masking of the narrow width based on mask information in the time domain causes overshoot/undershoot distortion in the vicinity of both edges of the frequency-domain signal effective band of a Fast Fourier Transform (FFT) output. In particular, because channel values used in the initial Inverse FFT (IFFT) process have zero values in the both sides of the guard band interval, they cause considerable dispersion around the time domain CIR component. In this respect, the sharp CIR masking increases edge distortion. The edge distortion ultimately results in error in Likelihood Log Ratio (LLR) calculation in the demapping process. Consequently, the decoding performance of the receiver is ultimately degraded.